1. Field of the Invention
The present invention relates to an apparatus for acquiring the responsibility of an oxygen concentration sensor. In the present invention, as an oxygen concentration sensor, a well-known electromotive force-type oxygen concentration sensor (well-known concentration cell-type oxygen concentration sensor using stabilized zirconia) is supposed. The output value of this oxygen concentration sensor comes to the first predetermined value (minimum value) when the air-fuel ratio is leaner than the theoretical air-fuel ratio (lean air-fuel ratio), and comes to the second predetermined value (maximum value) when the air-fuel ratio is richer than the theoretical air-fuel ratio (rich air-fuel ratio), and rapidly changes from the first predetermined value to the second predetermined value when the air-fuel ratio changes from the lean air-fuel ratio to the rich air-fuel ratio, and rapidly changes from the second predetermined value to the first predetermined value when the air-fuel ratio changes from the rich air-fuel ratio to the lean air-fuel ratio.
2. Description of the Related Art
Conventionally, an air-fuel ratio control unit comprising a three-way catalyst disposed in an exhaust channel of an internal combustion engine, and an oxygen concentration sensor disposed below the three-way catalyst in the exhaust channel is widely known. A technology to perform the so-called active control, for example, in order to calculate the maximum oxygen storage capacity of a three-way catalyst, by using this unit is widely known (refer to Japanese Patent Application Laid-Open (kokai) No. 2009-127597).
The active control refers to a control to compulsively dissociate the air-fuel ratio above the catalyst from the theoretical air-fuel ratio, and typically a control to switch the air-fuel ratio above the catalyst from the rich air-fuel ratio to the lean air-fuel ratio based on the fact that the output value of this oxygen concentration sensor is inverted from the first predetermined value to the second predetermined value and to switch the air-fuel ratio above the catalyst from the lean air-fuel ratio to the rich air-fuel ratio based on the fact that the output value of this oxygen concentration sensor is inverted from the second predetermined value to the first predetermined value. Namely, the air-fuel ratio above the catalyst is set alternately at the rich air-fuel ratio and the lean air-fuel ratio periodically. By this execution of the active control, the maximum oxygen storage capacity of a three-way catalyst can be calculated every time when the air-fuel ratio above the catalyst is switched.
By the way, due to the degradation of the oxygen concentration sensor or the like, the responsibility of the oxygen concentration sensor may decrease (the delay in response may increase). When the responsibility of the oxygen concentration sensor decreases, the time period required for the output value of this oxygen concentration sensor to be inverted from the first predetermined value to the second predetermined value (or conversely) becomes longer. As the result of this, the fluctuation cycle of the air-fuel ratio during the execution of the active control becomes longer. This means that the acquired maximum oxygen storage capacity of a three-way catalyst is calculated to be larger than its actual amount. Accordingly, in order to accurately calculate the maximum oxygen storage capacity of a three-way catalyst, it is necessary to acquire the responsibility of the oxygen concentration sensor.
In order to acquire the responsibility of the oxygen concentration sensor, various techniques have been proposed. For example, in Japanese Patent Application Laid-Open (kokai) No. 61-170643, the time period required for said inversion when the output value of this oxygen concentration sensor to be inverted from said second predetermined value (maximum value) to said first predetermined value (minimum value) is detected and, based on this detection result, the responsibility of the oxygen concentration sensor is acquired.